In general, the present invention relates to a multilayer wiring structure of a semiconductor device. In particular, the present invention relates to aluminum like metal wiring for semiconductor devices.
With high integration of semiconductor devices, the dimension rule gets thinner and thinner and it becomes necessary to connect upper-layer wires to lower-layer wires wiring with a wiring material through via holes and contact holes with a high aspect ratio in a wiring process. The contact holes and via holes are referred to hereafter simply as connection holes, a generic name for both.
One of the technologies for creating wires embedded into the connection holes described above is the sputtering technique. In general, a technique of embedding a refractory metal to a connection hole by means of the CVD (the abbreviation of Chemical Vapor Deposition) method is adopted. The CVD method is an excellent chemical vapor phase growth method for getting a metal embedded into a connection hole with a high aspect ratio.
In the process technology, the chemical vapor phase growth method of a refractory metal is adopted. One of known processes is a selected CVD process which adopts a combination of a blanket CVD method and an etch-back method. With this technique, a so-called plug is created in a connection hole by getting a refractory metal embedded into the connection hole.
In addition, the so-called barrier metal layered structure is adopted. In the barrier metal layered structure, a conductive refractory metal or compound such as titanium (Ti), titanium nitride (TiN), titanium oxide-nitride (TiON) and titanium tungsten (TiW) is created on the upper-surface and lower-surface sides of the aluminum (Al) like wire. In the barrier metal layered structure, a redundancy effect prevents the entire wiring from being broken even if the Al like wire itself is broken.
On the other hand, there is a method by which an upper-layer wire material is embedded in a connection hole. To be more specific, an Al reflow technology has been developed for flowing the upper-layer wiring material into a connection hole through heat treatment during or after a process of growing an Al like metal film. It should be noted that, in some cases, the technique of flowing the upper-layer wiring material into a connection hole during the growing process is called a high-temperature sputtering method while the technique of flowing the upper-layer wiring material into a connection hole after the growing process is referred to as the Al reflow method. In the following description, none the less, both the techniques are referred to as the Al reflow method.
With the technology for getting an Al like metal embedded into a connection hole by means of the Al reflow method, the Al like metal is heated to a temperature higher than the recrystallization temperature of the Al like metal but lower than the fusing point of the Al like metal during or after the growing process in order to flow the Al like metal. It should be noted that the recrystallization temperature varies from compound to compound but it is normally higher than 350.degree. C. Then, the flowing Al like metal is directed to get embedded into the connection hole.
In a wiring structure wherein a plug is created from a refractory metal embedded in a connection hole by a CVD method, however, voids result due to an electromigration (EM) phenomenon.
This phenomenon is studied and reported in documents such as the Proceedings of the 30th IEEE International Reliability Physics Symposium, 1992, Pages 338 to 343 authored by Jiang Tao et al.
An example of the void generation is explained by referring to FIGS. 1 to 3. As shown in FIG. 1, lower-layer wires 111 and 112 made of an Al like metal and an upper-layer wire 113 also made of an Al like metal are connected to each other by plugs 117 and 118 made of a refractory metal created inside connection holes 115 and 116 which are created through an interlayer dielectric film 114 between the lower-layer wires 111 and 112 and the upper-layer wire 113. In such a wiring structure, a current flows for example from the lower-layer wire 112 to the upper-layer wire 113 through the plug 118 and then from the upper-layer wire 113 to the lower-layer wire 111 through the plug 117. In the figure, the paths of electrons e.sup.- are indicated by arrows. As shown in the figure, the electrons e.sup.- flow from the lower-layer wire 111 to the upper-layer wire 113 through the plug 117 and then from the upper-layer wire 113 to the lower-layer wire 112 through the plug 118. Since the electrons e.sup.- flow through the plugs 117 and 118, EM (electromigration) phenomena occur therein. Accordingly, Al migrates from the upper-layer wire 113 in contact with the plug 117 and the lower-layer wire 112 in contact with the plug 118. For this reason, the amounts of Al at those portions become insufficient, resulting in voids 121 and 122 therein. As a result, the upper-layer and lower-layer wires 113 and 112 are broken, causing reliability to deteriorate.
On the other hand, voids also result from an EM phenomenon even in a barrier metal layered structure.
The generation of such voids is explained as follows. As shown in FIG. 2, main portions of lower-layer wires 111 and 112 are made of an Al like metal 131 whereas a main portion of an upper-layer wire 113 is made of an Al like metal 132. The lower-layer wires 111 and 112 and the upper-layer wire 113 are connected to each other by plugs 117 and 118 made of a refractory metal created inside connection holes 115 and 116 which are created through an interlayer dielectric film 114 between the lower-layer wires 111 and 112 and the upper-layer wire 113. Barrier metal layers 133 and 134 each made of a refractory metal or a compound are created on the upper surfaces of the lower-layer wires 111 and 112 respectively. On the other hand, a barrier metal layer 135 made of a refractory metal or a compound is created on the lower surface of the upper-layer wires 113. In this barrier metal layered structure, a wire is not broken easily due to an EM phenomenon in comparison with the wiring structure shown in FIG. 1. None the less, a current flows through the Al like metals 131 and 132 which each have a low resistance.
For example, a current flows from the lower-layer wire 112 to the upper-layer wire 113 through the plug 118 and then from the upper-layer wire 113 to the lower-layer wire 111 through the plug 117. In this case, electrons e.sup.- flow from the lower-layer wire 111 to the upper-layer wire 113 through the plug 117 and then from the upper-layer wire 113 to the lower-layer wire 112 through the plug 118. Such a current causes EM deterioration to occur at the upper-layer wire 113 in contact with the plug 117 and the lower-layer 112 in contact with the plug 118, resulting in voids 123 and 124 therein. As a result, reliability deteriorates.
In addition, in a wiring structure wherein an Al like metal is embedded into a connection hole by using the Al reflow technique, voids are also generated as well due to an EM phenomenon.
The generation of voids in a wiring structure built by means of the Al reflow technique is explained by referring to FIG. 3. As shown in the figure, lower-layer wires 111 and 112 made of an Al like metal and an upper-layer wire 113 also made of an Al like metal are connected to each other by connection holes 115 and 116 which are created through an interlayer dielectric film 114 between the lower-layer wires 111 and 112 and the upper-layer wire 113. In addition, in order to improve the wettability between the upper-layer wire 113 and the underlying interlayer dielectric film 114 as well as to enhance the EM-proof characteristic and the endurance characteristic against stress induced voiding of the upper-layer wire 113, an adhesion layer (including a barrier metal) 141 is normally created beneath the upper-layer wire 113. Typically, the adhesion layer is made of titanium (Ti) or titanium nitride (TiN). Much like the configuration shown in FIG. 8, for example, a current flows from the lower-layer wire 112 to the lower-layer wire 111 through the upper-layer wire 113. In such a case, EM deterioration occurs at the upper-layer wire 113 inside the connection hole 115 and at the lower-layer wire 112 beneath the connection hole 116, resulting in voids 125 and 126 therein. As a result, reliability deteriorates.
In general, an EM phenomenon occurs when electrons collide with metal atoms, in this case, Al atoms, causing the atoms to migrate due to an electron wind force (or electrons' colliding force). If EM phenomena occur uniformly over the entire wiring structure, the atom concentration in the wiring structure itself does not change, generating no voids therein. In actuality, however, a portion in which EM phenomena do not occur uniformly exists in the wiring structure.
In a portion in which there is a biggest difference in the number of migrating atoms, that is, in a portion in which the number of departing atoms is largest in comparison with the number of incoming atoms, a failure can be said to occur due to an EM phenomenon.
In each of the connection holes shown in FIGS. 1 to 3, more than one types of metals each with a high fusing point other than the Al like metal or a compound of such metals exist, being crossed by the current path. Thus, a boundary surface, where electrons flow out from the metals with a high fusing point or a compound of such metals to the Al like metal layer, has a most insufficient number of Al atoms. As a result, an EM failure occurs therein.
FIGS. 4-1 to 4-4 show Al-atom flows caused by an EM phenomenon. As shown in FIG. 4-1, an atomic current flows for example in a direction indicated by an arrow. As shown in FIG. 4-2, a portion with an insufficient number of Al atoms is depletion. A depletion region is caused by migration of atoms from the depletion region to a destination called an accumulation region, resulting in a difference in Al-atom concentration between the Al like metals at the depletion and accumulation regions. The difference in Al-atom concentration in turn gives rise to a stress gradient as is shown in FIG. 4-3. For this reason, a force works to push back Al atoms in the reverse direction from the accumulation region with an excess amount of Al metal to the depletion region with an insufficient amount of Al metal. The stress gradient induces an atomic current in the reverse direction as shown in FIG. 4-4. The flow of Al atoms in the reverse direction is normally called a back flow. Thus, the final EM phenomenon must be considered to be attributed to a net atomic current which is equal in quantity to the difference between the atomic flow caused by the force of the electron wind (that is, the colliding force of electrons) and the back flow.
FIG. 5-1 is a diagram roughly showing a cross section of a wiring structure whereas FIG. 5-2 is a rough layout diagram thereof. On a boundary surface between an Al like metal 211 and a refractory metal 213 above a connection hole 213, a stress gradient which is developed toward a region in the Al like metal 211 with an insufficient number of Al atoms exits in an area outside the current path. It should be noted that electrons e.sub.- flow in a direction opposite to that of the current. As a result, Al atoms are supplied to the region with an insufficient number of Al atoms from all the surroundings of the portion due to this stress gradient. In addition, the area of the Al like metal 211 outside the current path, in which this stress gradient is developed, has only a length of the order of w1, an aligment margin for the exposure apparatus. Accordingly, a void 214 is observed in the Al like metal 211 during examination of the surroundings of the connection hole after an EM test.